Dipolar interactions, molecular flexibility, and flexoelectricity in bent-core liquid crystals

General liquid-crystal theory for anisotropically shaped molecules: Symmetry, orientational order parameters, and system free energy

A general theory of liquid crystals is presented, starting from the group-theory symmetry analysis of the constituting molecules. A particular attention is paid to the type of elastic free-energies and their relationships with the molecular symmetries. The orientational order-parameter tensors are identified for each molecular symmetry, in a consideration of consistently keeping the leading, characteristic elastic free energies in a model. The order parameters are expressed in terms of symmetric traceless tensors, some of high orders, for all major molecular symmetries, including seven groups of axial symmetries and seven groups of polyhedral symmetries. For spatially inhomogeneous liquid crystals, the couplings of these tensors in the elastic energies are derived by expanding the interaction energies between these molecules. The aim is to provide a general view of the molecular symmetries of individual molecules, orientational order parameters characterizing the orientational distribution functions, and the elastic free energies, all under one single group-theory approach.

Stimulated transformation of soft helix among helicoidal, heliconical, and their inverse helices

Dynamic modulation of soft helix in terms of the molecular organization, handedness, and pitch length could result in a sophisticated control over its functions, opening numerous possibilities toward the exploration of previously unidentified applications. Here, we report a dynamic and reversible transformation of a soft helical superstructure among the helicoidal (molecules orthogonal to helical axis), heliconical (molecules oblique to the helical axis, i.e., oblique helicoidal), and their inverse helices, together with a tunability on the helical pitch, by combining electrical and optical manipulations. This multistate transformation depends on a matching of the temperature, the strength of external stimuli, and the bend and twist elastic effects of the system. A laser emission with tunable wavelength and polarization, and prescribed micropatterns formed by any aforementioned architectures were achieved.

Entropy-Driven Chiral Order in a System of Achiral Bent Particles

Why should achiral particles organize into a helical structure? Here, using theory and molecular dynamics simulations we show that at high concentration crescent-shaped particles interacting through a purely repulsive potential form the twist-bend nematic phase, which features helical order of the twofold symmetry axes of particles, with doubly degenerate handedness. Spontaneous breaking of the chiral symmetry is driven by the entropic gain that derives from the decrease in excluded volume in the helical arrangement. Crucial to this purpose is the concave shape of particles. This study is based on a general formulation of the Onsager theory, which includes biaxiality and polarity of phase and particles, in addition to the space modulation of order. Molecular dynamics simulations corroborate the theoretical predictions and provide further insights into the structure of the helical phase.

Molecular simulation study of polar order in orthogonal bent-core smectic liquid crystals

We explore the phase behavior and structure of orthogonal smectic liquid crystals consisting of bent-core molecules (BCMs) by means of Monte Carlo molecular simulations. A simple athermal molecular model is introduced that describes the basic features of the BCMs. Phase transitions between uniaxial and biaxial (antiferroelectric) orthogonal smectics are obtained. The results indicate the presence of local in-plane polar correlations in the uniaxial smectic phase. The macroscopic uniaxial-biaxial transformation is rationalized in terms of local polar correlations giving rise to polar domains. The size of these polar domains grows larger under the action of an external vector field and their internal ordering is enhanced, leading to field-induced biaxial order-disorder transitions.

Statistical mechanics of splay flexoelectricity in nematic liquid crystals

We develop a lattice model for the splay flexoelectric effect in nematic liquid crystals. In this model, each lattice site has a spin representing the local molecular orientation, and the interaction between neighboring spins represents pear-shaped molecules with shape polarity. We perform Monte Carlo simulations and mean-field calculations to find the behavior as a function of interaction parameters, temperature, and applied electric field. The resulting phase diagram has three phases: isotropic, nematic, and polar. In the nematic phase, there is a large splay flexoelectric effect, which diverges as the system approaches the transition to the polar phase. These results show that flexoelectricity can be a statistical phenomenon associated with the onset of polar order.

On the threshold characteristics of the flexoelectric domains arising in a homogeneous electric field: The case of anisotropic elasticity

Precise solutions for the threshold voltage U(c) and wave number q(c) that feature the appearance of longitudinal flexoelectric domains of Vistin'-Pikin-Bobylev at strong anchoring have been derived. Based on the formulated expressions, we present and analyze computer calculations for a planar nematic layer with anisotropic elasticity and both negative and positive dielectric anisotropy under the action of a homogeneous flexoelectrically deforming d.c. electric field. The obtained relations allow a selection of particular values of physical parameters, in order to improve the performance of devices exploiting flexoelectrcity in nematics.

Molecular simulation study of self-assembly of tethered V-shaped nanoparticles

We use Brownian dynamics to investigate the self-assembly of single end tethered, laterally tethered, and double end tethered V-shaped nanoparticles. The simulation results are compared with model bent-core molecules without tethers and polymer tethered nanorods to elucidate the combined effects of V-shaped geometry and the immiscibility between the V-shaped nanoparticles and the tethers on the self-assembled structures. We show that the V-shaped geometry significantly alters the phase diagram of tethered nanoparticles and further that the immiscibility between particles and tethers leads to structures not previously predicted for bent-core molecules. Examples of mesophases predicted include honeycomb, hexagonally packed cylinders, and perforated lamellar phases.

Computer simulations of biaxial nematics

Biaxial nematic (N(b)) liquid crystals are a fascinating condensed matter phase that has baffled, for more than thirty years, scientists engaged in the challenge of demonstrating its actual existence, and which has only recently been experimentally found. During this period computer simulations of model N(b) have played an important role, both in providing the basic physical properties to be expected from these systems, and in giving clues about the molecular features essential for the thermodynamic stability of N(b) phases. However, simulation studies are expected to be even more crucial in the future for unravelling the structural features of biaxial mesogens at the molecular level, and for helping in the design and optimization of devices towards the technological deployment of N(b) materials. This review article gives an overview of the simulation work performed so far, and relying on the recent experimental findings, focuses on the still unanswered questions which will determine the future challenges in the field.

Molecular dynamics investigation of the effects of a water surface on the aggregation of bent-core molecules

Molecular dynamics simulations are used to determine how the presence of a water surface affects the way that bent-core surfactant molecules interact with one another. The simulations are carried out for isolated pairs of bent-core molecules, and for pairs of bent-core molecules on a water surface. The results show that the water surface fundamentally alters the nature of the interaction between the bent-core molecules: a stable complex is formed when the two molecules are on the water surface, but not for an isolated pair of molecules. This difference occurs because the water surface constrains the internal structure and orientation of the molecules, which makes the packing of the molecules into a stable complex more thermodynamically favorable.

Density functional theory and molecular dynamics investigations on substituted banana-shaped compounds

Density functional theory (DFT) calculations and molecular dynamics (MD) simulations on the atomic level were performed on three different substituted banana-shaped compounds derived from 1,3-phenylene bis[4-(4-n-hexyloxyphenyliminomethyl)benzoate] (P-6-O-PIMB). The DFT studies were carried out on the isolated molecules, and in the MD simulations clusters were treated with up to 64 monomers. The effect of polar substituents, such as chlorine and the nitro group, on the central 1,3-phenylene unit of banana-shaped compounds was investigated. In particular, flexibility, polarity, electrostatic potential (ESP) group charge distributions, B-factors, bending angles and molecular lengths were considered. The MD results were analysed by trajectories of significant torsion angles as well as order parameters such as radial atom pair distribution functions g(r), orientational correlation functions g(o), diffusion coefficients (D) and root mean square deviations (RMSD) values. The g(r) and g(o) values show that a certain long range order is generated by the introduction of a NO(2) group in the 2-position of the central 1,3-phenylene ring. In contrast, the chlorination at the 4 and 6 positions of the central 1,3-phenylene unit decreases the long range order tendency by its perturbation effect on the conformations in such molecules. Moreover, g(r) and g(o) values, as well as diffusion coefficients, show that in the NO(2) substituted compound the formation of microphase areas is preferred. Finally, the aggregation effect in such compounds was studied in a systematic way by a comparison of the conformational properties of the isolated molecules and the monomers in the clusters.

Molecular dynamics investigation of bent-core molecules on a water surface

Molecular dynamics simulations are carried out for bent-core molecules at water surfaces. The water surface is shown to alter the equilibrium molecular structure significantly by causing a different class of torsional states to become more favorable. The equilibrium structure is also altered by the substitution of chlorine atoms for hydrogen atoms on the central phenyl ring in that this substitution forces the bent core to remain in a single torsional state rather than be delocalized among several torsional states. The consequences of these structural changes on the chirality and packing of these molecules on water surfaces are discussed.

Giant flexoelectricity of bent-core nematic liquid crystals

Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals (NLCs) via the electric current produced by periodic mechanical flexing of the NLC's bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy.

Orientational ordering of a banana-shaped solute molecule in a nematic calamitic solvent by 2H-NMR spectroscopy: an indication of glasslike behavior

The Saupe ordering matrix of a banana-shaped mesogenic molecule as a solute in a common nematic calamitic solvent has been determined by 2H-NMR spectroscopy as a function of temperature. The temperature dependence of the Saupe ordering matrix element associated with the principal molecular axis is consistent with a glassy behavior in the reorientational motion of this particular solute molecule. The Haller expression, appropriately modified, provides a good fit to the experimental data.